1. Field of the Invention
The invention relates to an improved electrical machine having at least four exciter poles in the stator and having a commutator rotor with a greater number of slots and pole teeth on its circumference than the number of exciter poles.
2. Description of the Prior Art
From German Patent DE 197 57 279 C1, it is known, in a quadrupole electric motor, to use a commutator rotor with twelve commutator laminations and twelve coils connected to them, in order to attain low torque ripple and good commutation. The diametrically opposed laminations are joined together via contact bridges, in order to make the power supply to the rotor symmetrical and to assure it with only one pair of brushes. The contact bridges are produced upon winding of the coils by first winding one coil from one lamination and then, via the adjacent lamination, with the coil wire laying a contact bridge to the diametrically opposite lamination and then from there in alternation to produce the further coils and contact bridges. The rotor coils are produced in pairs on diametrically opposed sides with two so-called flyers, and the contact bridges are then formed by rotating the rotor 180°. In such machines, especially for small structural sizes, it is quite complicated to wind the high number of coils into the slots of the rotor. Moreover, the coils, wound around three pole teeth, intersect at the winding caps, which makes for a large protrusion of the winding caps. Moreover, because of the high winding pitch over three slots, long winding cap connections result, which are expensive in terms of material and also lead to high heat losses.
From U. S. Pat. No. 4,532,449, an electrical machine with a commutator rotor is also known, in which the number of rotor coils is only half as large as the number of commutator laminations. In this reference, five coils are supplied from one pair of brushes via ten laminations. The coils are continuously wound as single-tooth windings, by skipping one pole tooth from one coil to the next. The beginning and end of the coils are each contacted with laminations between each of which one lamination remains free. These free laminations, for supplying current to the coils, are connected via contact bridges to the laminations diametrically opposite them, which are in contact with the coils. This provision has the disadvantage that because of the two carbon brushes offset from one another by 90°, the commutation events are chronologically staggered, and as a result the commutating coils on the one hand cause radially acting forces, which can lead to noise production as well as unilateral loads on the rotor bearing, with correspondingly greater bearing wear. Moreover, because of increased lamination tension when there are ten instead of twelve laminations, increased brush fire occurs, which can impair the service life of the commutator and hence also the service life of the machine.
With the present solution to this problem, the object, in electrical machines whose number of rotor coils is only half as large as the number of laminations, is to improve the commutation and make it possible to produce the electrical machines, even in small structural sizes, without coil intersections at the winding caps.